1. Field of the Invention
The present invention relates to a random access scheme of a user equipment in a mobile communication system, and more particularly, to a random access scheme for preventing unnecessary retransmission and a user equipment for the same.
2. Discussion of the Related Art
As an example of a mobile communication system to which the present invention is applicable, a 3rd Generation Partnership Project Long Term Evolution (3GPP LTE) communication system will be schematically described.
FIG. 1 is a schematic diagram of an Evolved Universal Mobile Telecommunications System (E-UMTS) network architecture as an example of a mobile communication system.
The E-UMTS is an evolved version of the existing UMTS and basic standardization thereof is in progress under the 3GPP. The E-UMTS is also referred to as a Long Term Evolution (LTE) system.
The E-UMTS network may be roughly divided into an Evolved UMTS Terrestrial Radio Access Network (E-UTRAN) 101 and a Core Network (CN) 102. The E-UTRAN 101 generally comprises a terminal (i.e., User Equipment (UE)) 103, a base station (i.e., eNode B or eNB) 104, an Access Gateway (AG) 105 that is located at an end of the E-UMTS network and connects with one or more external networks. The AG 105 may be divided into a part for processing user traffic and a part for handling control traffic. Here, an AG for processing new user traffic and an AG for processing control traffic can communicate with each other using a new interface.
One eNode B may have one or more cells. An interface for transmitting the user traffic or the control traffic may be used among the eNode Bs. The CN 102 may comprise an AG 105, nodes for user registration of other UEs 103, and the like. An interface may be used to distinguish the E-UTRAN 101 and the CN 102 from each other.
The various layers of the radio interface protocol between the terminal and the network may be divided into Layer (L1), Layer 2 (L2) and Layer 3 (L3), based upon the lower three layers of the Open System Interconnection (OSI) standard model that is well-known in the field of communications systems. Among these layers, Layer 1 (L1), namely, the physical layer, provides an information transfer service by using a physical channel, while a Radio Resource Control (RRC) layer located in Layer 3 (L3) performs the function of controlling radio resources between the terminal and the network. The RRC layer exchanges RRC messages between the terminal and the network. The RRC layer may be located by being distributed in network nodes such as the eNode B 104, the AG 105, and the like, or may be located only in the eNode B 104 or the AG 105.
FIGS. 2 and 3 show an architecture of a radio interface protocol between a terminal and a UTRAN according to the 3GPP radio access network standard.
The radio interface protocol shown in FIGS. 2 and 3 is horizontally composed of a physical layer, a data link layer, and a network layer, and is vertically composed of a user plane for transmitting user data and a control plane for transferring control signaling. In detail, FIG. 2 shows the layers of the radio protocol control plane and FIG. 3 shows the layers of the radio protocol user plane. The protocol layers of FIGS. 2 and 3 may be divided into L1 (Layer 1), L2 (Layer 2), and L3 (Layer 3) based upon the lower three layers of the Open System Interconnection (OSI) standard model that is widely known in the field of communication systems.
Hereinafter, particular layers of the radio protocol control plane of FIG. 2 and the radio protocol user plane of FIG. 3 will be described.
The physical layer (PHY) (Layer 1) provides an information transfer service to an upper layer using a physical channel. The PHY layer is connected to a Medium Access Control (MAC) layer located thereabove via a transport channel, and data is transferred between the PHY layer and the MAC layer via the transport channel. At this time, the transfer channel is roughly divided into a dedicated transfer channel and a common transfer channel depending on whether or not a channel is shared. In addition, data is transferred respectively between different physical layers, namely, between the respective physical layers of the transmitting side and the receiving side via a physical channel using radio resources.
Various layers are located in Layer 2. First, the Medium Access Control (MAC) layer maps various logical channels to various transfer channels and performs a logical channel multiplexing function for mapping various logical channels to one transfer channel. The MAC layer is connected to a Radio Link Control (RLC) layer which is an upper layer via a logical channel, and the logical channel may be roughly divided into a control channel for transmitting information about the control plane and a traffic channel for transmitting information about the user plane, according to the type of transmitted information.
The RLC layer of the second layer segments and concatenates data received from an upper layer, thereby controlling a data size so as to be suitable for a lower layer to transmit data to a radio interval. The RLC provides three modes, namely, a transparent mode (TM), an unacknowledged mode (UM) and an acknowledged Mode (AM) to support various QoSs requested by each radio bearer (RB). Especially, for reliable data transmission, the AM RLC performs a function to retransmit data through an automatic repeat request (ARQ).
A packet data convergence protocol (PDCP) layer located at the second layer is used to efficiently transmit IP packets, such as IPv4 or IPv6, on a radio interval with a relatively narrow bandwidth. For this purpose, the PDCP layer reduces the size of an IP packet header which is relatively great in size and includes unnecessary control information, namely, performs a function called header compression. Accordingly, only necessary information can be included in the header part of data for transmission, so as to increase a transmission efficiency of a radio interval. In the LTE system, the PDCP layer also performs a security function. The security function includes a ciphering function for preventing data monitoring from a third party, and an integrity protection function for preventing data manipulation from a third party.
A radio resource control (RRC) layer located at a highest portion of the third layer is defined in the control plane. The RRC layer handles logical channels, transport channels and physical channels for the configuration, re-configuration and release of radio bearers. Here, a radio bearer (RB) denotes a logical path provided by the first and second layers of radio protocols for data transfer between the terminal and the UTRAN. Generally, configuration of the RB indicates a process of regulating radio protocol layers and channel characteristics necessary for providing a specific service, and configuring specific parameters and operation methods. The RB is divided into a signaling RB (SRB) and data RB (DRB). The SRB is used as a path through which an RRC message is transmitted on a C-plane, while the DRB is used as a path through which user data is transmitted on a U-plane.
Downlink transport channels for transmitting data from a network to a terminal may include a Broadcast Channel (BCH) for transmitting system information and a downlink Shared Channel (SCH) for transmitting other user traffic or control messages. Traffic or control messages of a downlink multicast or broadcast service may be transmitted either via a downlink SCH, or via a separate downlink Multicast Channel (MCH). In addition, uplink transport channels for transmitting data from a terminal to a network may include a Random Access Channel (RACH) for transmitting an initial control message and an uplink Shared Channel (SCH) for transmitting user traffic or control messages.
Downlink physical channels for transmitting information transferred to a downlink transport channel via a radio interval between a network and a terminal may include a Physical Broadcast Channel (PBCH) for transmitting BCH information, a Physical Multicast Channel (PMCH) for transmitting MCH information, a Physical Downlink Shared Channel (PDSCH) for transmitting PCH information and downlink SCH information, and a Physical Downlink Control Channel (PDCCH) (also called a DL L1/L2 control channel) for transmitting control information sent from the first and second layers, such as downlink or uplink radio resource allocation information (DL/UL Scheduling Grant) or the like. Uplink physical channels for transmitting information transferred to an uplink transport channel via a radio interval between a network and a terminal may include a Physical Uplink Shared Channel (PUSCH) for transmitting uplink SCH information, a Physical Random Access Channel (PRACH) for transmitting RACH information, and a Physical Uplink Control Channel (PUCCH) for transmitting control information sent from the first and second layers, such as HARQ ACK or NACK, Scheduling Request (SR), Channel Quality Indicator (CQI) report and the like.
The HARQ operation performed in the LTE system based on the above description will now be described.
FIG. 4 is a diagram showing a HARQ operation performed in the LTE system.
In FIG. 4, description will be given in an uplink state in which a UE is a transmission side, a base station (eNode B or eNB) is a reception side, and HARQ feedback information is received from the base station, but may be equally applied to downlink transmission.
First, the eNB may transmit uplink scheduling information, that is, uplink grant (UL grant), via a Physical Downlink Control channel (PDCCH), in order to enable the UE to transmit data using the HARQ scheme (S401). The UL grant may include a UE identifier (e.g., C-RNTI, semi-persistent scheduling C-RNTI), a location of an assigned radio resource (resource block assignment), a transmission parameter such as a modulation/coding rate, a redundancy version and the like, a new data indicator (NDI), etc.
The UE may check UL grant information sent to itself by monitoring a PDCCH in each Transmission Time Interval (TTI). In case of discovering the UL grant information sent to itself, the UE may transmit data (data 1 in FIG. 4) via a physical uplink shared channel (PUSCH) according to the received UL grant information (S402). In this case, the transmitted data can be transmitted by a MAC Protocol Data Unit (PDU).
As described above, after the UE has performed the uplink transmission via the PUSCH, the UE waits for reception of HARQ feedback information via a Physical Hybrid-ARQ Indicator Channel (PHICH) from the eNB. If HARQ NACK for the data 1 is transmitted from the eNB (S403), the UE retransmits the data 1 in a retransmission TTI of the data 1 (S404). On the contrary, if HARQ ACK is received from the eNB (not shown), the UE stops the HARQ retransmission of the data 1.
Each time the UE performs one data transmission using the HARQ scheme, the UE takes a count of the number of transmissions (CURRENT_TX_NB). If the transmission number reaches a maximum transmission number (CURRENT_TX_NB) set by an upper layer, the UE discards the MAC PDU stored in a HARQ buffer.
If the HARQ ACK for the data 1 retransmitted in the step S404 from the UE is received (S405) and if a UL grant is received via the PDCCH (S406), the UE may determine whether data to be transmitted this time is an initially-transmitted MAC PDU or whether to retransmit a previous MAC PDU using a new data indicator (NDI) field received via the PDCCH. In this case, the NDI field is a 1-bit field. The NDI field is toggled as 0→1→0→1→ . . . each time a new MAC PDU is transmitted. For the retransmission, the NDI field is set to a value equal to that of the initial transmission. In particular, the UE may determine whether to retransmit the MAC PDU, by comparing the NDI field with a previously transmitted value.
In case of FIG. 4, as a value of NDI=0 in the step S401 is toggled into NDI=1 in the step S406, the UE recognizes that the corresponding transmission is a new transmission. The UE may transmit data 2 via a PUSCH (S407).
Meanwhile, a procedure of, at a UE, performing random access to an eNB will now be described.
First, the UE may perform a random access procedure in the following cases:                when the UE performs initial access because there is no RRC Connection with the eNB,        when the UE initially accesses a target cell in a handover procedure,        when the random access procedure is requested by a command of a base station,        when there is uplink data transmission in a situation where uplink time synchronization is not aligned or where a specific radio resource used for requesting radio resources is not allocated, and        
when a recovery procedure is performed in case of a radio link failure or a handover failure.
In the LTE system, there are two procedures in selecting a random access preamble: one is a contention based random access procedure in which the UE randomly selects one preamble within a specific group for use, another is a non-contention based random access procedure in which the UE uses a random access preamble allocated only to a specific UE by the eNB. The non-contention based random access procedure may be used, as described above, only in the handover procedure or when it is requested by the command of the eNB.
Meanwhile, a procedure in which a UE performs random access to a specific eNB may include steps of (1) at the UE, transmitting a random access preamble to the eNB (hereinafter, referred to as a “first message (Message 1)” transmission step), receiving a random access response from the eNB in correspondence with the transmitted random access preamble (hereinafter, referred to as a “second message (Message 2)” reception step), (3) transmitting an uplink message using information received by the random access response message (hereinafter, referred to as a “third message (Message 3)” transmission step), and (4) receiving a message corresponding to the uplink message from the eNB (hereinafter, referred to as a “fourth message (Message 4)” reception step).
FIG. 5 shows an operation procedure between a UE and an eNB in a contention based random access procedure.
(1) First Message (Message 1) Transmission
First, a UE may randomly select a random access preamble within a group of random access preambles indicated through system information or a handover command, may select PRACH resources capable of transmitting the random access preamble, and then may transmit the selected random access preamble (Step 501).
(2) Second Message (Message 2) Reception
After transmitting the random access preamble in step S501, the UE may attempt to receive a response with respect to its random access preamble within a random access response reception window indicated through the system information or the handover command by the eNB (Step S502). More specifically, the random access response information is transmitted as a MAC PDU, and the MAC PDU may be transferred via the Physical Downlink Shared Channel (PDSCH). In addition, the Physical Downlink Control Channel (PDCCH) may be monitored such that the terminal appropriately receives information transferred via the PDSCH. That is, the PDCCH may include information about a UE that should receive the PDSCH, frequency and time information of radio resources of the PDSCH, a transfer format of the PDSCH, and the like. Here, if the PDCCH has been successfully received, the UE may appropriately receive the random access response transmitted via the PDSCH according to information of the PDCCH. The random access response may include a random access preamble identifier (ID) (e.g., Random Access Preamble Identifier (RAPID)), a UL Grant indicating uplink resources, a temporary C-RNTI, a Time Advance Command (TAC), and the like.
Here, the random access preamble identifier is included in the random access response in order to notify UEs to which information such as the UL Grant, the temporary C-RNTI, and the TAC would be valid because one random access response may include random access response information for one or more UEs. Here, it is assumed that the random access preamble identifier may be identical to the random access preamble selected by the UE in Step 502. Accordingly, the UE may receive the UL Grant, the temporary C-RNTI and the TAC.
(3) Third Message (Message 3) Transmission
If the UE has received the random access response valid to the UE itself, the UE may process each of the information included in the random access response. That is, the UE applies the TAC, and stores the temporary C-RNTI. In addition, data to be transmitted may be stored in a Message 3 buffer in correspondence with the reception of the valid random access response.
In addition, the UE uses the received UL Grant so as to transmit data (that is, Message 3) to the eNB (Step S503). Message 3 should be included in the identifier of the UE. This is because, in the contention based random access procedure, the eNB may not determine which UEs are performing the random access procedure, but later the UEs should be identified for contention resolution.
Here, two different schemes may be provided to include the UE identifier. A first scheme is to transmit the UE's cell identifier through an uplink transmission signal corresponding to the UL Grant if the UE has already received a valid cell identifier allocated in a corresponding cell prior to the random access procedure. Conversely, the second scheme is to transmit the UE's unique identifier (e.g., S-TMSI or random ID) if the UE has not received a valid cell identifier prior to the random access procedure. In general, the unique identifier is longer than the cell identifier. If the UE has transmitted data corresponding to the UL Grant, the UE starts a Contention Resolution (CR) timer.
(4) Fourth Message (Message 4) Reception
After transmitting the data with its identifier through the UL Grant included in the random access response, the UE waits for an indication (instruction) of the eNB for the contention resolution. That is, the UE attempts to receive the PDCCH so as to receive a specific message (Step 504). Here, there are two schemes to receive the PDCCH. As described above, if the UE identifier included in Message 3 transmitted in correspondence with the UL Grant is the cell identifier, the UE attempts to receive the PDCCH by using its own cell identifier. If the UE identifier included in Message transmitted in correspondence with the UL Grant is its unique identifier, the UE attempts to receive the PDCCH by using the temporary C-RNTI included in the random access response. Thereafter, for the former, if the PDCCH is received through its cell identifier before the contention resolution timer expires, the UE determines that the random access procedure has been successfully (normally) performed, thus completing the random access procedure. For the latter, if the PDCCH is received through the temporary cell identifier before the contention resolution timer expires, the UE checks data transferred by the PDSCH that the PDCCH indicates. If the unique identifier of the UE is included in the data, the UE determines that the random access procedure has been successfully (normally) performed, thus completing the random access procedure.
Meanwhile, if the contention resolution procedure through the transmission of Message 3 and the reception of Message 4 has not been successfully performed, the UE may select another random access preamble so as to restart the random access procedure. To this end, the UE may receive Message 2 from the eNB, configure Message 3 for contention resolution procedure, and transmit Message 3 to the eNB. The HARQ process used for the transmission of Message 3 in the HARQ system which was described with reference to FIG. 4 may be different from the HARQ process for the transmission of Message 3 in the previous random access attempt. In this case, there may be a problem in which the MAC PDU stored in the HARQ buffer corresponding to the previous HARQ process may be unnecessarily retransmitted. The present inventors provide a technology for recognizing and solving the above problem.